Magnetron oscillators



April 17, 1956 w. E. wlLLsHAw MAGNETRON OSCILLATORS 5 Sheets-Sheet lFiled Feb. 26, 1952 FIG. 3.

INVENTOR h/l/ L/qM ERNEST M//U-SHYV TTORNEY April 17 1956 w. E. wlLLsHAw2,742,570

l MAGNETRON OSCILLATORS Filed Feb. 26, 1952 5 Sl'leecs-Sheeil 2 3 ll l 2FIG 2 I6 INVENTOR M//LLm/vf ERNEST l'V/L/.s/qw

HTTORNEY April 17, 1955 w. E. wlLLsHAw 2,742,570

MAGNETRON OSCILLATORS Filed Feb. 25, 1952 5 SheetS-She. 5

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FITTORNEY UnitedStates Patent C MAGNETRON oscrrLArronsl f WilliamErnest: Willshaw,.Kenton,v England, assigner to The M70 Valve. CompanyLimite.dLondon, Englandv Application February-26, 1952, Serial No.273,430

Claims, priority application. Great Britain February 28, 1951' 8-Claims. (Cl, Z50-36) This.. invention relates to magnetron oscillatordevices of the multiple resonant cavity type. comprisingametal blockanode formedwith a cylindrical electron orbit hole withinwhich lies, atherrnionic cathode and having a p1urality of resonant cavities recessedintoy the metal block, the cavities being spaced round the cathode andopening on to.; the electronorbit hole. A

Such` magnetron devices are adapted to.. co-operate. withmagnetic'elds.` forv the generation, of electrical oscillations and are.especially.l suitable, for the generation of oscillationsv of; very highfrequency. ln some cases it is desirable toV arrange that the frequencyofthe oscillations generatedcanbe varied during operation oftheoscillator, and mechanical tuning means. for effecting such frequencychanges are well know-n; meansy for effecting the changes electronicallyhave also been usedv or proposed, such means in general comprising meansfor producing a stream or cloud of electrons. with which the electromag;netic. teldof theresonant system of the magnetron oscillator caninteract, the interaction resulting in a change in the operatingfrequency of the oscillator.

The object of the invention is tov provide any improved form ofmagnetron oscillator device of the type specified provided with meansfor effecting an electronic control of the operating frequency of theoscillator.

According to the invention a multiple resonant cavity an auxiliarythermionic cathode located within one of the said resonant. cavities andis provided with leads suitable for energizing andl supplying operatinglcurrent to the auxiliary magnetron diode formed by the co-operation ofthe metal block anodeand said auxiliarycathode.

We have found that with the construction in accord ance with theinvention, when the magnetron oscillator is operated under the normalconditions with the auxiliary diode not energised, the normal operatingfrequency is obtained and there need be little loss of output poweroccasioned by the presence of the diode; but if the auxiliary diode isthen energised with anode-cathode voltage difference below the cut-offvalue for .such a diode, the operating frequency ofthe oscillator isdecreased and the frequency shift produced can be varied by increasingor decreasing the current taken by the diode. We believe that. thisfrequency dependence is due to the formation of an electron cloudaroundl the auxiliary cathode ofthe diode, which cloudy acts as arellector to electromagnetic waves imp/inging: on its surface andmodifies the .electrostatic capacity of the resonator system; variationof the diode current results in a change in the cloud size and hence inthe electrostatic capacity of the resonator system, withresulting changein the frequency of operation of the oscillator.

The change in `operating frequency .of the lmagnetron oscillator isaccompanied by some loss .of Eoutput power.,

f but we have found that-by-careful design the lossl of power can belimited to relatively small values, .for example vless thanforminirnisingvloss of power itis `,de-

l magnetron oscillator device of the type specified includes` Y `and2,742,570 Patentedv Apr. 17, l9 5 6 ICC sirableI for the uniformity otthe magnetic lieldi with which the magnetron'. oscillator deviceco-operatesA to extendi not only across, the electron orbit hole butalso? to the cavity. includingY the aumliary. cathode, and.` for thisreason: magnetic pole-pieces of somewhaty greater area thanv usual` aredesirable withv the arrangement in accord.- ance with the invention; itis alsodesirable that the auxili- Viary cathode shoulda' bey accuratelycentr-.odin the cavity Vin Y which itis. situated' and` that itshould=be slottedy to mini.-

anode with their axesfparallel to. thev axis of: the electron orbithole, and the auxiliary thermionic cathode provided in accordance withthe invention is then preferably. also cylindrical and arrangedf withits axisv parallel tothe axis off the electron orbit hole. Where theaxiliary cathode lieawithin a part of a-v cavity which isf substantiallyof circular cross-section, the auxiliary cathode isl preferably also of.substantially circular cross-section and coaxial therewith.

The manner of operation of the auxiliary diode in. a magnetronoscillator device in accordance with the invenf tion will. depend on theuseto which it is to be put. Thus, for example, iiY a simple shift fromone value to another in the operating frequency of the Voscillator isrequired, the auxiliary cathode may be merely arrangedl to be connectedto theanodethrough a switch, closingv of thel switch'in. operationv ofthe-*oscillator connecting the auxiliary cathode directlyA tof theanode, .thus rendering; the auxiliary magnetron diode operative andreducing the operating frequency of thevoscillator; for obtaining'anincreased frequency drop it may be arranged that the diode current is'.increased by underloading the magnetron oscillator and/ or by increasingthe anode-cathode voltage of the diode, but this will also furtherdecrease the output power. v Y

It will be appreciated` that the frequency changes obtained with amagnetronV oscillator device in accordance with the invention arerelated to variations in the other parameters of the oscillator and tothe operating conditions of the diode and that all such factors must betaken into account and may be taken advantageof in applying themagnetron oscillator device to any particular use.

The invention will be further described with reference to one particularembodiment, ywhich is given by-'wayof example and is illustratedy in theaccompanying drawing iny which Y 'Y Figure l shows a perspective view ofthe magnetronv oscillator device, partly cut away to reveal the internaldetails, Y

Figure 2 shows an end View of thedevicewith oneend plate removed, f f

Figure 2(11) shows a' detail of the reverse side of Figure. 2, Figure 3shows a section'in the plane A-A of Figure 2,

Figures: 4, 5. and 6 show schematically different circuit arrangementsfor the v.operation .of the magnetron oscillator. device shown inFigures l to 3.

-Referringniow .to Figures 1 to 3, the magnetron oscillator device shownycomprises a cylindrical copper block 1 recessed from .each end face 2to leave a .central part-i tion ,3. which .forms the'anode proper, thispartition being perforated by .a cylindrical hole 4 which forms 'theelectron .orbit hole and .by .eight resonantwcayities (5, .6) which.open into the Velectron orbit holeand are spaced symmetr ally roundits. axis.; .each .cavity is in .the form 'of a cylindrical hole. -5;haras its axis parallel to, the axis 3 of the electron orbit hole andcommunicating with that hole through a narrow radial slot 6.

A cylindrical indirectly heated thermionic cathode 7, provided withcircular end discs 8 in the usual manner, is supported within andcoaxial with the electron orbit hole by two stout conductors 9 lying oneon each side of the central partition and insulatingly sealed throughthe curved wall of the block through glass-to-metal thimble sidearmseals 10 in the usual way. The cathode is insulated from one of thediscs 8 for enabling current to be passed through the cathode heaterfrom the conductors 9, in the usual way. Also one of the resonant cavityholes is ttted with an output coupling loop 11, the lead 12 to whichpasses along a radial slot extending from the said cavity hole to theexterior of the block and enters into a glass-to-metal thimble sidearrn13 closing the slot, the end 14 of the lead being sealed through theglass thimble closure 15 of the sidearm in known manner.

Alternate anode segments between the resonant cavities are strappedtogether on each side of the anode par tition 3 by wire coupling straps16 in a known system of strapping utilising two straps on one side ofthe block, as shown in Figure 2, and four straps on the other side ofthe block, as indicated in Figure 2a, in which only the details requiredto indicate the strapping have been shown. The ends of the metal blockare closed by circular copper end plates 17 attached to the block bygold washer seals in known manner (one of the plates being omitted inFigures l and 2), and metal cooling tins are soldered to the exterior ofthe block in the completed device; these ns have been omitted from thedrawings in order not to obscure the other details of structure.

The construction so far described does not differ from a knownconstruction for a magnetron oscillator device adapted to operate at awavelength of say, 10 cms. In accordance with the invention, however,one of the resonant cavities is provided with an auxiliary thermioniccathode, and for convenience this cavity is chosen as the next-but-oneto that containing the coupling loop. A cylindrical indirectly heatedauxiliary thermionic cathode 18, also provided with end discs 19, issupported coaxially within the circular hole part of the said cavity bytwo stout conductors 20 lying one on either side of the central anodepartition 3 and insulatingly sealed through the adjacent curved wall ofthe block through glass-to-metal thimble sidearm seals 21 similar to,and lying at right angles to, the sidearm seals 10 for the main cathode,the conductors 20 providing the cathode and heater leads for theauxiliary cathode in the same way as the conductors 9 provide thecathode and heater leads for the main cathode.

The auxiliary thermionic cathode consists of a hollovl tube 22 withinwhich lies the heater 23, the outer surface of the tube being coatedwith electron emissive material (not indicated) in the usual way, andthe tube is cut by a narrow slot 24 (see Figure l) extending the wholelength of the cathode tube parallel to its axis. The tube 22 isinsulated from one end disc 19 for enabling current to be passed throughthe heater 23 from the conductor 20.

In one form of this construction, designed for normal operation at awavelength of about 10 centimetres, the magnetron block had thefollowing dimensions:

Mms.

(a) Diameter of electron orbit space 16 (b) Axial length of electronorbit space 20 (c) Diameter of main cathode 6 (d) Radial depth of theparallel-sided slot part of the resonant cavities 3.3

(e) Width of said slot part (d) 2.2

(f) Diameter of the circular hole part of said resonant cavities 10 (g)Diameter of auxiliary cathode 5 When this magnetron oscillator devicewas pulse operated in a magnetic field of 1735 oersted, produced betweenpolepieces of 3 inches diameter, the anode to main cathode voltage being17,000 volts, the anode current being 25 amps, the pulse repetition ratebeing 500 per second and the pulse duration being l microsecond, it wasfound that with the auxiliary cathode not energised and left oatingf thenominal output frequency was obtained and the efficiency was about 63%.When the auxiliary cathode was connected directly to the anode theoutput 'frequency decreased by about 2 mc./s. and the efciency fell byabout 5%.

When the frequency shift was increased to 5 mc./s. by increasing thediode current by applying a biassing voltage between its anode andcathode, the efficiency fell by about a further 11%.

This use of the magnetron oscillator device is illustrated schematicallyin Figure 4 of the accompanying drawings, in which the device isrepresented only by the anode block 1, and in which for simplicity theheating circuits for the main and auxiliary cathodes, and the means forapplying the magnetic eld, have all been omitted.

The main operating circuit for the magnetron device is represented bythe modulator M connected between the anode 1 and cathode 7 for theapplication of negative voltage pulses to the cathode 7, and by thewaveguide load W connected to the output coupling loop 11. The auxiliarythermionic cathode 18 is connected to the anode through a switch S, anda source of bias voltage B, which is omitted if a simple switchingaction only is required.

Alternative ways of energising the auxiliary diode are illustrated inFigures 5 and 6, in which parts corresponding to those in Figure 4 areindicated by the same reference numerals as in that gure.

In the arrangement of Figure 5, the auxiliary thermionic cathode 18 isconnected to the negative terminal of the modulator M through the switchS and a resistor R, which may be for example of about 14,500 ohmsresistance for a magnetron device having the dimensions and operatingconditions quoted.

In the arrangement of Figure 6, the auxiliary cathode 18 is connectedthrough the switch S to the junction of a resistor (TiTz) connectedbetween the output terminals of the modulator M; for a magnetron devicehaving the dimensions and operating conditions quoted, the values of T1and T2 may be, for example, respectively 6950 ohms and 6000 ohms.

In some circuit arrangements it may be desirable to maintain theauxiliary magnetron diode introduced in accordance with the inventionpermanently operative. Thus with a magnetron device in accordance withthe invention as shown in Figures l to 3, if the oscillator anodecurrent is varied, it is found that over a wide range of at least thehigher values of oscillator anode current the frequency shift resultingfrom the presence of the diode decreases as the anode current increases.In pulsed operation this effect may be obtained with either of thearrangements shown `in Figures 5 and 6 by shorting out the switch 5 andvarying the modulator output voltage, the frequency shifts, and thechanges in the frequency shifts, being different in the two cases.

Such variations of the frequency shift can be arranged to reduce to aconsiderable extent the frequency pushing experienced with magnetronoscillator devices of the type specified in operation, that is to saythe change in operating frequency which occurs when the oscillator anodecurrent is varied from a given value, as is sometimes required. Thus ifthe operating frequency of a magnetron oscillator device of the typespecified not provided with an auxiliary cathode in accordance with theinvention is plotted against the oscillator anode current, it is foundthat the curve exhibits a maximum, the frequency decreasing withvariation of the anode current ineither `direction from a particularvalue ofanode current. But when provided with an auxiliary cathodeforminga magnetron diode connected asv shown inFigure or Figure 6 withthe switchesl shorted out andthe resistor R,or resistors Tiand T2, ofsuitable resistance, it can y,

be arranged that for .values of the oscillator anode current above thesaid particular value, the change in the frequency shift with change inthe `oscillator anode current compensates for the frequency pushingelect` to a large extent.

For operation of the magnetron oscillator device in accordance with theinvention below the said particular value of oscillator anode current, abiassing control of the diode anode-cathode voltage is required toensure that the diode current decreases as the oscillator anode currentincreases; in pulse operation this may be effected Y by applying areversed polarity pulse to the diode simulthe oscillacathode beingdisposed within one only of the saidV resonant cavities, and leads tothe auxiliary thermionic cathode for energizing and supplying operatingcurrent to the auxiliary magnetron diode formed by the cooperation ofthe said anode and. said auxiliarycathode. said auxiliary cathode beingslotted along its length for reducing the induction of circulatingcurrents therein in the operation of the device.

2. A magnetron oscillator device accordingto claim 1 wherein theresonant cavities are vformed by cylindrical slots traversing the metalblock anode with their axes parallel tothe electron orbit hole, .and theauxiliary p thermionic cathode is alsocylindrical and arranged with its`axis parallel to the axis of the electron orbit hole.

3. A magnetron oscillatorv device according to claim 2 wherein theauxiliary cathode is of substantially circular cross-section and isarranged coaxially within a part of a resonant cavity which part is ofsubstantially circular cross-section.

4. An electric circuit arrangement comprising a magnetr'on oscillatordevice in accordance with claim l in combination with means forenergising the main magnetron oscillator, and means for energising theauxiliary magnetron diode. at below the cut-oik voltage for the l diode,the last said means includingr switch means for alternatively making andbreaking the auxiliary magnetron diode anode-cathode circuit forlvarying the operating frequency of the magnetron oscillator.

`5. An electric circuit arrangement according Vto claim 4 comprising apulse modulator connected between the anode and cathode of themagnetrony oscillator device for the application of high voltageoperating pulses thereto, and wherein the said auxiliary thermioniccathode is connected to the output ofY the pulse modulator through thesaid switch means and through a resistance, for the appli-v cation'ofthe voltage pulses with reduced amplitude between the anode and cathodeof the auxiliary magnetron diode when said switch means is closed.

6. A ymagnetron oscillator device according to claim 1V wherein theauxiliary thermionic cathode is slotted parallel to its longitudinalaxis along the whole of its length.

7. A 'multiple resonant cavity magnetron oscillator device comprising ametal block anode having a circular cylindrical electron orbit hole, amain magnetron cathode in said orbit hole, said anode having a pluralityof resonant cavities recessed into the block, the cavities being spacedaround the main cathode and each consisting of a circular cylindricalslot whose vaxis is parallelvto the axisv of 'the i electron orbit holeand a r'adialslr'ot narrower-,than the diameter ofthe cylindrical slotconnecting the latter to the electron vorbitl hole, a single auxiliarythermionic cathode located wholly within a circular cylindrical slotlpart ofk only one resonant cavity, and leads to the auxiliary thermionickcathode' for energizing andsupplying operating current tothe auxiliarymagnetron Adiode formed by the cooperation of said anode and and saidauxiliary cathode. 8. A magnetron oscillator device according to claim.7 wherein the auxiliary thermionic cathode is circularly cylindricaland coaxial with the circular cylindrical slot part of the resonantcavity in which it is located.

References Cited the le of this patent UNITED S'IA'l-ES PA'IENTSy2,446,531 ADerby Aug. l0, 1948 2,504,739 Shoupp Apr. 18, 1950 2,617,079McNall Nov. 4, 1952

